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the free encyclopedia.
- For other uses, see
Life and
Living
Life is a multi-faceted
concept. Life may refer to
an ongoing process of which living
things are a part, the period
between the
conception (or a point at
which the entity can be considered
to be an individualized being) and
death of an
organism, the condition of an
entity that has been born (or
reached the point in its existence
at which it can be established to
be alive) and has yet to die, and
that which makes a living thing
alive.
Defining the concept of life
How can one tell when an entity
is a lifeform? It would be
relatively straightforward to
offer a practical set of
guidelines if one's only concern
were life on
Earth as we know it (see
biosphere), but as soon as one
considers questions about life's
origins on Earth, or the
possibility of extraterrestrial
life, or the concept of
artificial life, it becomes
clear that the question is
fundamentally difficult and
comparable in many respects to the
problem of defining
intelligence. Also, loosely
speaking, some theories are
grounded in the basic assumption
that
"ideas have a life of their own".
A conventional definition
Unsolved problems in biology:
How did life start? Is life
a cosmic phenomenon? Are the
conditions necessary for the
origin of life narrow or
broad? How did life originate
and diversify in hundreds of
millions of years? Why, after
rapid diversification, do
microorganisms remain
unchanged for millions of
years? Why have so many
biological systems developed
sexual reproduction? How do
organisms recognize like
species? How are the sizes of
cells, organs, and bodies
controlled?
In
biology, a lifeform has
traditionally been considered to
be a member of a population whose
members can exhibit all the
following phenomena at least once
during their
existence:
-
Growth, full development,
maturity
-
Metabolism, consuming,
transforming and storing
energy/mass;
growing by absorbing and
reorganizing mass; excreting
waste
-
Motion, either moving
itself, or having internal
motion
-
Reproduction, the ability to
create entities that are similar
to, yet separate from, itself
- Response to
stimuli - the ability to
measure properties of its
surrounding
environment, and act upon
certain conditions. This
property is also called
homeostasis.
Exceptions to the conventional
definition
These criteria are not without
their uses, but their disparate
nature makes them unsatisfactory
from a number of perspectives; in
fact, it is not difficult to find
counterexamples and examples that
require further elaboration. For
example, according to the above
definition, one could say:
-
Mules and people who are
infertile cannot reproduce
and thus would not qualify as
lifeforms. Also worker bees and
other organisms living in
colonies would not qualify; only
the queen and the drones (or the
whole colony) can be considered
'alive'.
-
Fire and
stars could be considered
lifeforms.
- A
virus does not grow and
cannot reproduce outside of a
host cell and thus would not
qualify as a lifeform.
Many individual organisms are
incapable of reproduction and yet
are still considered to be
lifeforms; see
mules and
ants for examples. This is
because the term "lifeform"
applies on the level of entire
species or of individual
genes. (For example, see
kin selection for
information about one way by which
non-reproducing individuals can
still enhance the spread of their
genes and the survival of their
species.) It is important to keep
in mind the difference between a "lifeform"
and "a being that is alive." One
example of
sterility does not render the
rest of the species a non-lifeform,
any more than one dead animal
renders the rest of the species
dead.
Note also that the two cases of
fire and stars fitting the
definition of life can be simply
remedied by defining metabolism in
a more biochemically exact way.
Fundamentals of Biochemistry
by
Donald Voet and
Judith Voet (ISBN
0471586501) defines metabolism
as follows: "Metabolism is the
overall process through which
living systems acquire and utilize
the free energy they need to carry
out their various functions. They
do so by coupling the exergonic
reactions of nutrient oxidation to
the endergonic processes required
to maintain the living state, such
as the performance of mechanical
work, the active transport of
molecules against concentration
gradients, and the biosynthesis of
complex molecules." This
definition, in use by most
biochemists, makes it clear that
fire is not alive, because fire
releases all the oxidative energy
of its fuel as heat.
(Note: Actually, the definition
does not help much at all. For it
is circular. What we are looking
for, after all, is a definition of
"living entity." We agreed that
part of the definition is "capable
of metabolism." We then tried to
define "metabolism" in order to
get clear on which entities are
capable of it and which not. But
the definition of "metabolism"
just offered is in terms of living
systems. And those are exactly
what we are trying to define!)
This could also be remedied by
adding the requirement of
locality, where there is an
obvious structure that delineates
the spatial extension of the
living being, such as a
cell membrane.
A conceptual problem with
saying that fire is life is that
it collapses the distinction
between "growth" and
"reproduction." It is possible to
think of a spreading flame as
either growing or reproducing, but
what would it mean to say that the
same act is both growth and
reproduction?
Viruses reproduce, flames grow,
some software programs mutate and
evolve, future software programs
will probably evince (even
high-order) behavior, machines
move, and proto-life, consisting
of metabolizing cells without
reproduction apparatus, can have
existed. Still, some would not
call these entities alive.
Generally, all five
characteristics are required for a
population to be considered a
lifeform.
Other definitions
Biologists who are content to
focus on terrestrial
organisms often note some
additional signs of life,
including these:
- Living organisms contain
molecular components such
as:
carbohydrates,
lipids,
nucleic acids, and
proteins.
- Living organisms require
both energy and matter in order
to continue living.
- Living organisms are
composed of at least one
cell.
- Living organisms maintain
homeostasis for some period
of time.
-
Species of living organisms
will
evolve.
All life on Earth is based on
the
chemistry of
carbon compounds. Some assert
that this must be the case for all
possible forms of life throughout
the universe; others describe this
position as 'carbon
chauvinism'.
The
systemic definition is that
living things are self-organizing
and
autopoietic (self-producing).
These objects are not to be
confused with
dissipative structures (e.g.
fire). Variations of this
definition include:
-
Francisco Varela and
Humberto Maturana's
definition of life (also widely
used by
Lynn Margulis) as an
autopoietic
(self-producing),
water based,
lipid-protein
bound,
carbon metabolic,
nucleic acid replicated,
protein readout
system
- "a system of inferior
negative feedbacks subordinated
to a superior positive
feedback" (J.
theor Biol. 2001)
- Tom Kinch's definition of
life as a highly organized
auto-cannibalizing system
naturally emerging from
conditions common on planetary
bodies, and consisting of a
population of replicators
capable of mutation, around each
set of which a homeostatic
metabolizing organism, which
actively helps reproduce and/or
protect the replicator(s), has
evolved
-
Stuart Kauffman's definition
of life as an
autonomous agent or a
multi-agent system capable
of reproducing itself or
themselves, and of completing at
least one
thermodynamic work cycle
-
Robert Pirsig's definition
of life, found in his book
Lila: An Inquiry into Morals,
as that which maximizes its
range of possible futures, in
other words, that which makes
decisions that result in the
most future choices, or that
which strives to keep its
options open.
- A
system converting
entropy to
negentropy, using flow of
energy.
Other definitions:
- That which seeks to continue
its own existence (attributed to
Clifford A. Schaffer).
Descent with modification: a
"useful" characteristic
A useful characteristic upon
which to base a definition of life
is that of
descent with modification: the
ability of a life form to produce
offspring that are like its parent
or parents, but with the
possibility of some variation due
to
chance. Descent with
modification is sufficient by
itself to allow
evolution, assuming that the
variations in the offspring allow
for differential survival. The
study of this form of heritability
is called
genetics. In all known life
forms (assuming
prions are not counted as
such), the genetic material is
primarily
DNA or the related molecule,
RNA. Another exception might
be the
software code of certain forms
of
viruses and programs created
through
genetic programming, but
whether
computer programs can be alive
even by this definition is still a
matter of some contention.
Origin of life
Main article:
Origin of life
There is no truly "standard"
model of the origin of life, but
most currently accepted models
build in one way or another on the
following discoveries, which are
listed roughly in order of
postulated emergence:
- Plausible pre-biotic
conditions result in the
creation of the basic small
molecules of life. This was
demonstrated in the
Urey-Miller experiment.
-
Phospholipids spontaneously
form
lipid bilayers, the basic
structure of a
cell membrane.
- Procedures for producing
random
RNA molecules can produce
ribozymes, which are able to
produce more of themselves under
very specific conditions.
There are many different
hypotheses regarding the path that
might have been taken from simple
organic molecules to protocells
and metabolism. Many models fall
into the "genes-first"
category or the "metabolism-first"
category, but a recent trend is
the emergence of hybrid models
that do not fit into either of
these categories.
The possibility of
extraterrestrial life
Main articles:
Extraterrestrial life,
Astrobiology
As of 2005,
Earth is the only planet in
the
universe known by humans to
support life. The question of
whether life exists elsewhere in
the universe remains open, but
analyses such as the
Drake equation have been used
to estimate the probability of
such life existing. There have
been a number of claims of the
discovery of life elsewhere in the
universe, but none of these have
yet survived scientific scrutiny.
Today, the closest that
scientists have come to finding
extraterrestrial life is fossil
evidence of possible
bacterial life on
Mars (via the
ALH84001 meteorite). Searches
for extraterrestrial life are
currently focusing on planets and
moons believed to possess liquid
water, at present or in the past.
Recent evidence from the NASA
rovers
Spirit and
Opportunity supports the
theory that Mars once had surface
water. See
Life on Mars for further
discussion.
Jupiter's moons are also
considered good candidates for
extraterrestrial life, especially
Europa, which seems to possess
oceans of liquid water.
Other highly speculative and
somewhat doubtful places for
present or past life include the
atmosphere of
Venus,
Titan cyrovolcanoes, or even
Enceladus.
See also
References
- Kauffman, Stuart. The
Adjacent Possible: A Talk with
Stuart Kauffman. Retrieved Nov.
30, 2003 from
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